While I sometimes see Dutch as an endangered language, reality is that with nearly 30 million speakers worldwide, we're actually doing pretty well. Sadly, this can't be said for the 3000 truly endangered languages of the world - nearly half of the world's total number of languages is on the verge of extinction, and with it, large amounts of human culture are in danger of disappearing forever. In collaboration with several universities and language institutions, Google has launched the Endangered Languages Project to document these languages - textually, visually, and
auditorially.

Hebrew yes. Cornish is more problematic. There are a number of competing versions. The only one I ever liked is the least popular, the "Modern" version by Richard Gendall. It's most like modern Welsh and Breton to me. The others are nice, but retain archaic and quirky features and spellings.

Ah, there-in lies the reason for the languages being endangered. As we move to a more global community, it's no longer practical be monolingual in a smaller language. If you're bilingual, you'll default to language spoken by more of your contacts, which will typically be the larger one. Thus the smaller one fades into obscurity and eventually dies.

I do not think we should allow any language to die without being thoroughly studied. But, the benefit of everyone speaking a common language is tremendous. IMHO, it's good to have people thinking in languages of different families since a Chinese-speaker and English-speaker may have unique perspectives, but cultural identity shouldn't trump progress. We shouldn't keep people speaking different languages just so those languages don't die.

... But, the benefit of everyone speaking a common language is tremendous.

I agree with this. (Although whether English is a good choice is another question.)

We shouldn't keep people speaking different languages just so those languages don't die.

I disagree with this. There are hundreds or thousands of years of history embodied in the language, idioms, stories, etc., which are lost if the language goes out of usage.

There are many groups of people in the world that are bilingual (a "native" language and a national/international one), and this seems to work very well. I think this is a model that should be encouraged.